The present invention relates to an exhaust gas cleaning device for a diesel engine, particularly to an exhaust gas cleaning device provided with electrical heating means for igniting and burning combustible particulates in exhaust gases.
The amount of the unburnt carbon particulates in exhaust gases discharged from a diesel engine is much larger than that discharged from engines powered with other fuels.
The number of the diesel cars has increased recently and several methods for reducing the unburnt carbon particulates have been proposed.
According to one of such methods, the unburnt carbon particulates are collected by a filter formed of a ceramic porous body or metallic fiber.
In this method, as the distance covered by vehicle increases, carbon particulates gradually accumulate on the filter and accordingly the pressure loss of exhaust gases increases.
Therefore, it is required to eliminate the accumulated carbon particulates and to revive the filter.
Namely, as the accumulation of carbon particulates on the surface of the filter proceeds, the flowing resistance of the filter increases so that the output of the engine lowers. And the mass of the collected particles begins to fall from the surface of the filter so that the operation of the filter lowers.
Since the particulates accumulated on the filter are mainly composed of carbon particulates containing a small amount of the fuel component, these particulates can be burnt and eliminated by heating them at a temperature not less than about 580.degree. C.
However, the temperature of the exhaust gases emitted from the diesel engine is much lower than that of the exhaust gases emitted from the gasoline engine and is not more than 580.degree. C. except at high speed.
Therefore, special methods are required for eliminating particulates from the filter.
One method for raising the temperature of the exhaust gases to the combustion temperature thereof is to increase the ratio of fuel to air by lowering the amount of intake air by means of a throttle valve.
According to this method, it is difficult to sufficiently raise the temperature of exhaust gases when the engine is running at a low speed.
U.S. Pat. No. 4,211,075 discloses a grid shaped electric heater which is disposed on the upstream side of a filter member for heating the exhaust gases.
According to this patent, the electric heater firstly heats the exhaust gases and the heated exhaust gases heat the filter member and the particulates collected thereby.
Therefore, heat applied by the electric heater is easily lost due to radiation loss.
And since the whole of the exhaust gases is heated, a large amount of electric power is required for heating the particulates collected by the filter member to the combustion temperature thereof.
Furthermore, the electric heater does not have a sufficiently high strength against vibrations.
Accordingly, one object of the present invention is to provide an exhaust gas cleaning device which cleans the exhaust gases emitted from a diesel engine by collecting and burning the carbon particulates in the exhaust gases.
Another object of the present invention is to provide an exhaust gas cleaning device by which most of the carbon particulates can be collected, and the collected carbon particulates can be burnt off by a small amount of electric power consumption.
Still another object of the present invention is to provide a durable exhaust gas cleaning device having a simple structure.